As is known, chemical sensors detect the presence of gas thanks to a chemical reaction which takes place between molecules of the gas and a sensitive film. The chemical reaction depends significantly on an operating temperature which influences the effects of adsorption, desorption and diffusion of the gas in the film. Consequently, temperature is an important factor in optimizing the performance of the sensors, particularly as regards sensitivity, selectivity and response time. To obtain an optimum operation, therefore, the sensors are provided with means for regulating and controlling temperature.
Recently, integrated chemoresistive gas microsensors, the manufacture of which makes use of microelectronics techniques, have been proposed and produced. These microsensors have the following advantages: reduced manufacturing costs, low energy consumption in operation, short response times and integrability with a temperature control and output signal processing circuit.
Integrated gas microsensors using chemoresistive films based on tin oxide have appeared on the market. On the surfaces of such films, deposited on a wafer of semiconductor material machined using a technique of "bulk micromachining", described below, a chemical reaction takes place between oxygen of the film and the gas to be detected which has an effect of changing the resistance of the film and thus enables the presence of the gas to be detected.
In view of the fact that in order to operate correctly, such sensors must be maintained at temperatures of approximately 400.degree. C., they are provided with heater elements and must be thermally insulated from the rest of the chip integrating a signal processing and control circuit.
Various techniques for isolating the sensitive film from the rest of the chip are known in literature. The technique used historically is that of "bulk micromachining", which includes producing the sensitive film on top of or inside a dielectric layer deposited on a massive silicon wafer and of removing a portion of massive silicon from the back of the wafer with wet etching methods. The dielectric layer performs a dual task of mechanically supporting the sensor and thermally insulating the sensor from the massive silicon wafer. In the context of this technique, prototypes have been produced with partial removal of the silicon from the area of the sensor, in which the excavation is carried out only on part of the thickness of the wafer, and prototypes which provide the total removal of the silicon in correspondence with the area of the sensor (the etching reaches as far as the dielectric layer carrying the sensor element). As regards this second solution, reference may be made for example to the article entitled "Basic Micro-Module for chemical sensors with on chip heater and buried sensor structure" by D. Mutschall, C. Scheibe, E. Obermeier.
On the other hand, the technique of bulk micromachining requires the presence of front-back machining processes and comprises particular demands for handling the chips which are such that it proves to be incompatible with current integrated circuit manufacturing methods.
Another proposed technique includes "front micromachining" on the basis of which the massive silicon wafer is etched from the front and a dielectric layer mechanically supports and thermally insulates the sensor element. In this respect, for manufacturing a different type of sensor, reference may be made for example to the article by D. Moser and H. Baltes entitled "A high sensitivity CMOS gas flow sensor based on an N-poly/P-poly thermopile", DSC-Vol. 40, Micromechanical Systems, ASME, 1992; furthermore, for a survey of the techniques of bulk and front micromachining, reference may also be made to the article entitled "Micromachining and ASIC technology" by Axel M. Stoffel in Microelectronics Journal, 25 (1994), pages 145-156.
In this case also, the technique for producing suspended structures requires the use of etching steps that are not very compatible with the current manufacturing processes used in microelectronics and does not therefore permit sensors and the related control and processing circuitry to be obtained on a single chip.
Furthermore, the use of dedicated SOI (Silicon-On-Insulator) substrates has been proposed, in which the starting wafer comprises a stack of silicon/silicon oxide/silicon, with the oxide selectively removed at the sensor area, forming an air gap. The excavations made from the front of the wafer at the end of the process steps to produce the air gap and enable the sensor to be thermally insulated. In this respect, for a shear stress sensor, reference may be made for example to the article by J. Shajii, Kay-Yip Ng and M. A. Schmidt entitled "A Microfabricated Floating-Element Shear Stress Sensor Using Wafer-Bonding Technology", Journal of Microelectromechanical Systems, Vol. 1, No. 2, June 1992, pages 89-94. A method used for bonding (apart from forming the air gap) is further described in the article "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J. Hausman, G. A. Spierings, U. K. P. Bierman and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, No. 8, August 1989, pages 1426-1443.